1. Field of the Invention
The present invention relates to a knife insert in accordance with the preamble of claim 1.
2. Description of the Prior Art
Logs to be sawn are usually worked at least on one side, however, most generally on two sides prior to rip-sawing. This operation is normally performed through milling certain side parts of the log into chips by passing the log into a gap formed by two revolving cutter heads. In the art are used knife heads shaped as a truncated cone and having knife inserts mounted thereto staggered in screw-thread fashion, whereby the side of the log will be chipped sequentially by each knife insert at a time starting from the outermost perimeter of the cutter head and proceeding radially toward the axis of revolution of the cutter head. Square timber hewing machines, chipping canters and similar wood-working machines generally have at least two cutter heads with a number of knife inserts attached to their perimeter in a spiralling succession so as form a screw thread. One such knife head is described in U.S. Pat. No. 4,499,934. Therein, each of the knife inserts mounted on the knife head has two cutting edges, of which one is arranged to cut approximately with the grain of stock (i.e., approximately longitudinally with the grain in at least some plane perpendicular to the working direction), while the other edge cuts obliquely to the grain. The knife insert is mounted on the knife head by its heel, which is formed by the extension of the with-the-grain cutting edge toward the center axis of the knife head. Such a knife insert has been developed further by aligning the obliquely-cutting edge at an outward pointing angle with respect to the with-the-grain cutting edge. One embodiment of this type of knife insert is described in a data sheet HewSaw R250 released by Veisto-Rakenne Rautio Oy, Finland.
Knife inserts of the prior-art technology have, however, been found to be problematic in cases where the knife insert runs in a small radius of rotation cutting the grain at an unfavourable angle. Such a situation is encountered when, e.g., a knife insert mounted on a small-diameter knife head is used for working large trunks. Then, the with-the-grain cutting edge starts to flex the cut chip outward before the cross-the-grain cutting edge has cut the wood grain. Resultingly, the chips will be of different sizes, which causes lower quality of the chips and other defects.